1. Field of the Invention
This invention relates to a stereo ultradirectional microphone apparatus for receiving and converting a sound into a set of stereo sound signals.
2. Description of the Prior Art
Sets of stereo microphones are known. As a simple, a set of stereo microphones comprising two directional microphones are used. Each of these directional microphones has a unidirectional characteristic showing a high sensitivity in a direction (hereinafter this direction in which the microphone shows a high sensitivity is referred to as a main lobe). Two directional microphones are arranged to obtain a stereo effect such that a lobe of one directional microphone is directed to +.theta. direction and a lobe of the other directional microphone is directed to -.theta. direction with respect to the front thereof wherein .theta. is selected from the range 45.degree..ltoreq..vertline..theta..vertline..ltoreq.90.degree.. Such general type stereo microphones aim to record sounds from sources existing in a wide angle range viewed from the recording point, i.e., a location of the stereo microphones. However, if a sound from a source existing a predetermined narrow angle range is recorded using general type of stereo microphones, it is impossible to record the sound with a sufficient SN ratio because such stereo microphones have too large width of the main lobe, so that sounds coming from directions other than the predetermined narrow angle rage are recorded as noises. In the actual recording scene, such situations may occur frequently. As a solution to this problem, in place of the unidirectional microphone, an ultradirectional microphone having a more sharp directional characteristics is studied to be applied to the directional microphone apparatus (GERLACH H, "Stereo sound recording with shotgun microphones", J Audio Eng Soc, Vol. 37 No. 10 Page 832-838 '89). This document discloses examples of a stereo recording apparatus to which the ultradirectional microphones is applied, namely, XY and MS structures. The XY structure has two ultradirectional microphones are used where one is directed in +.theta. direction and the other is directed in -.theta. direction with respect to the front thereof on recording.
The MS structure has one ultradirectional microphone and a hi-directional microphone wherein a main lobe of the ultradirectional microphone is directed to the front and the lobe of the hi-directional microphone is directed to have an angle of 90.degree. from the front. Left side and right side outputs are obtained by adding or subtracting between the outputs of these two microphones. Both XY and MS structures provide the recording of a sound from a source existing in the more narrow angle range than the general stereo microphones. That is, these structures provide the stereo recording of a sound from a more remote sound source because there is a tendency that unnecessary sounds are not mixed with the necessary sound. In other words, assuming the distances between the sound source and the microphones are the same, these structure provide the stereo recording with a higher SN ratio. However, the document reports problems as follows:
In the XY structure, a sound having a high frequency from a sound source existing at left or right side with respect to the microphones is left and the sound existing at the center is suppressed. Contrary, in the MS structure, the higher frequency of a sound, the more the stereo feeling is lost.